1. Field of the Invention
The present invention relates generally to thermal isolation, and more particularly to thermal isolation in radio frequency (RF) transmission lines coupled to cooled systems.
2. Related Art
Any radio frequency (RF) conductor, such as a cable or waveguide, that includes a metallic component conducts heat. When such an RF conductor is used for connection to a cooled system, heat is transmitted to the cooled system through the RF conductor. The result is a loss of cooling in the cooled system, an increase in the power needed to maintain the desired temperature in the cooled system, or both.
One example of a cooled system is a transceiver placed in a dewar cryogenically cooled by liquid nitrogen to approximately 77 degrees Kelvin. By employing high temperature superconductivity (HTS) technology, such systems can achieve reductions in weight, size and RF loss. One potential application for such an HTS transceiver is in a cellular telephone base station, where there is a demand for a low-noise high-performance front end. Another potential application for an HTS transceiver is on board a communications satellite, where there are similar requirements.
One approach to achieving thermal isolation is to simply cut a gap in the transmission line. While this approach provides excellent thermal isolation, it unfortunately also produces large ohmic signal loss.
Another approach is to use very thin transmission lines to reduce heat flow through the transmission lines. While this approach provides moderate thermal isolation, it also produces moderate signal loss. Further, such transmission lines are unreliable due to their fragility.
The present invention is a radio frequency (RF) thermal isolator and method of manufacture for same. According to one embodiment, the RF thermal isolator includes a first transmission line; a second transmission line of nominally the same dimensions as the first transmission line and axially aligned with the first transmission line, wherein the ends of the transmission lines are separated by a gap having a width that is a very small fraction of the center operating wavelength at the operating frequency of the transmission lines; and an electrically conductive sleeve electrically attached to the end of the first transmission line and surrounding the end of the second transmission line and separated from the second transmission line by a gap having a width that is a very small fraction of the center operating wavelengths at the operating frequency of the transmission lines; wherein the sleeve extends along the second transmission line from the end of the first transmission line for a distance of nominally xc2xc of the center operating wavelength at the operating frequency of the transmission lines.
In one aspect the gaps have a width that is nominally {fraction (1/100)} of the center operating wavelength at the operating frequency of the transmission lines.
In one embodiment, each of the transmission lines is a waveguide. In another embodiment, each of the transmission lines is a coaxial cable having an inner conductor and an outer conductor. A center conductor extends axially from the inner conductor of the first transmission line into a cavity in the center conductor of the second transmission line, wherein the center conductor extends beyond the end of the first transmission line for a length that is nominally xc2xc of the center operating wavelength at the operating of transmission lines. The cavity extends into the center conductor of the second transmission line for a distance of nominally xc2xd of the center operating wavelength of the transmission lines.
In one aspect the RF thermal isolator includes a mechanical coupler attached between the transmission lines.
In one aspect the transmission lines and sleeve are fabricated from a conductive metal.
In one aspect the transmission lines and sleeve are fabricated from a composite material coated with a metallic layer.
In one aspect the inner conductors of the coaxial cables are hollow, and the cavities within the RF thermal isolator are vented to each other and to the exterior of the RF thermal isolator.
The method of manufacture includes electrically attaching an electrically conductive sleeve upon the outer surface of a first transmission line, wherein the sleeve extends beyond an end of the first transmission line for a distance of nominally xc2xc of the center operating wavelength at the operating frequency of the first transmission line, and disposing an end of a second transmission line of nominally the same dimensions as the first transmission line within the sleeve such that the second transmission line is axially aligned with the first transmission line and the ends of the transmission lines are separated by a gap having a width that is a very small fraction of the center operating wavelength at the operating frequency of the transmission lines; wherein the sleeve surrounds the end of the second transmission line and is separated from the second transmission line by a gap having a width that is a very small fraction of the center operating wavelength at the operating frequency of the transmission lines.
According to one embodiment, each of the transmission lines is a waveguide.
According to another embodiment, each of the transmission lines is a coaxial cable having an inner conductor and an outer conductor, and the method includes forming a cavity in the center conductor of the second transmission line, the cavity having a length of nominally xc2xd of the center operating wavelength at the operating frequency of the transmission lines; and mounting a center conductor upon the inner conductor of the first transmission line such that the center conductor extends axially from the inner conductor of the first transmission line into the cavity in the center conductor of the second transmission line, wherein the center conductor extends beyond the end of the first transmission line for a length that is nominally xc2xc of the center operating wavelength at the operating frequency of the transmission lines.
In one aspect the method includes mounting a mechanical coupler between the transmission lines.
In one aspect the method includes mounting a mechanical coupler between the sleeve and the second transmission line.
In one aspect the method includes mounting a retainer upon the second transmission line; and mounting a mechanical coupler between the sleeve and the retainer.
In one aspect the transmission lines and sleeve are fabricated from a conductive metal.
In one aspect the transmission lines and sleeve are fabricated from a composite material coated with a metallic layer.
In one aspect the inner conductor of the coaxial cables is hollow, and the cavities within the coaxial cables and the sleeve are vented to each other and to the exterior of the RF thermal isolator.
In one aspect the gaps have a width that is nominally {fraction (1/100)} of the center operating wavelength at the operating frequency of the transmission lines.
According to one embodiment, the present invention includes the product made by the process of the methods described above.
One advantage of the present invention is that it provides excellent thermal isolation with minimal signal loss.
Further features and advantages of the present invention as well as the structure and operation of various embodiments of the present invention are described in detail below with reference to the accompanying drawings.